The present invention relates to prosthetic ball and joint sockets for the human hip and particularly to a trial head and insert for an implantable hip joint prosthesis. The head and insert are unique in that they are self-locking to the the ball end of an intramedullary stem and yet can be instantly and easily released therefrom without the need for any tools.
Prosthetic ball and socket joints for surgical human hip repair have been known since about 1948. During the early years of this surgical joint repair, only the ball was replaced, the artificial ball being seated in the natural acetabular socket of the hip. Experience showed that friction of the artificial ball against the natural acetabular socket was unsatisfactory, causing wear of the acetabulum bearing surface and resulting in excessive progressive pain and stiffening of the hip joint.
The next phase of development resulted in the surgical procedure known as total hip replacement wherein not only the femoral head was replaced by a prosthesis but also the acetabular socket was enlarged and an acetabular prosthetic socket implanted. The natural femoral head is replaced by a metal intramedullary stem which terminates in a collar portion and neck portion with an offset ball or head portion which has a highly polished spherical bearing surface. The prosthetic acetabular socket component is usually in the form of a concave, cup-shaped, socket member shaped to receive the ball-shaped head of the femoral component. The bearing surface of the acetabular socket component is usually made of a synthetic resin such as an ultra-high molecular weight polyethylene. The combination of a highly polished metal surface bearing against a smooth durable synthetic resin provides for low-friction long-lasting joint operation.
While these total hip replacement prostheses have been quite successful, the surgical procedure involved is very complicated and time consuming. Because of the length of time required for the operation, the risk of infection and complication is very high and the surgical trauma is excessive, particularly for elderly patients. For these reasons, there have been recent developments which suggest that partial hip replacements are preferred for an increasing percentage of hip procedures. Averill, U.S. Pat. No. 3,863,273, describes one of these improved partial prosthetic hip implant joints having a femoral intramedullary stem unit with a reduced size spherical metal ball which can freely rotate within a plastic bearing insert which is locked within a metal head which seats within the natural acetabular socket. This device is recommended for use when fractures of the femoral neck or other conditions of the femur occur but in which the function of the acetabulum is unaffected. Not only is this partial hip joint replacement a faster, easier and less traumatic surgical procedure but it also results in a quicker return to full mobility for the patient. This modern partial hip replacement prosthesis works much better than the earlier prosthetic joints because most of the hip movement occurs by rotation of the metal femoral ball within the spherical socket of the bearing insert. If excessive joint motion is required, beyond the capability of the femoral ball/bearing insert, then the outer metal head rotates slightly within the natural acetabular socket, to accommodate the excessive joint motion.
Since the partial hip implant utilizes the natural acetabular socket, many sizes of prosthetic heads must be available to simulate the natural femoral head and insure a close fit between the natural acetabular socket and the prosthetic head. Prosthetic heads for partial hip implants are commonly available with spherical radii of 20.6 mm, 21.4 mm, 22.2 mm, 23.0 mm, 23.8 mm, 24.6 mm, 25.4 mm, 26.2 mm and 26.9 mm.